1. Field of the Invention
The present invention relates generally to a method and apparatus for delivering radiation therapy. More particularly, the present invention relates to a method and apparatus for delivering radiation therapy during suspended ventilation.
2. Discussion
Radiation for the treatment of cancer embodies a variety of risks related to overexposure to healthy tissue. A major concern in increasing the dose to treat cancer is the potential increase in life-threatening complications. This is particularly the case for treatment in the thoracic and upper abdominal regions. Because of respiratory motion, a large margin is needed to ensure proper tumor coverage, which in turn leads to a large volume of healthy tissue being irradiated. For lung treatment, there is a risk of five percent pneumonitis in five years if the whole lung receives more than 1,750 cGy, two-thirds of the lung receives more than 3,000 cGy, and one-third of the lung receives 4,500 cGy. Similar observations have been made for other sites, such as the treatment of focal lesions in the liver.
There are rather difficult tolerances to satisfy if one wants to increase dose. Take, for example, the traditional radiation treatment using AP/PA (anterior to posterior/posterior to anterior) beam arrangements for lung treatment. Given, for example, a modest lung thickness of 15 cm. Assuming a total lung capacity of 5.0 liters, the total irradiated lung volume is calculated by taking the lung volume around the tumor and subtracting tumor volume. Given a margin of 3 cm around the tumor that is 7 cm in diameter, 45% of the lung will initially be irradiated. Given a margin of 2 cm, 30% of the lung will be irradiated. Given a margin of 1 cm, 18% of the lung will be irradiated. Given a margin of 0.5 cm, 13% of the lung will be irradiated.
In response to concerns regarding over-exposure, there have been intense efforts over the past decade to implement high dose conformal radiation therapy which have led to the development of many new advanced technologies. These advanced technologies include computed tomographic (CT) simulation, three dimensional (3D) treatment planning, computer controlled medical accelerators, multileaf collimators (MLCs), and electronic portal imaging devices (EPIDs). These technologies are becoming increasingly more common, making possible the implementation of new treatment techniques such as intensity modulated radiation therapy. The success of high dose conformal therapy depends critically on treatment accuracy. With more accurate information about the position of a tumor, a tighter treatment margin can be prescribed such that a higher dose can be delivered to the tumor without increasing deleterious complications.
In practice, the treatment margin must account for the width of the beam penumbra, the daily variation in patient setup, and the variation in organ positions between fractions and during a single fraction. Recent advances have been made to sharpen beam penumbra, reduce daily setup variation and compensate for inter-fraction variation of organ position. (Intra-fraction organ motion associated with breathing, however, remains problematic.) Intra-fraction variations pertain to the changes in the organ shapes and positions during a single treatment fraction. These include the motion of tumors and organs in the thoracic and abdominal regions. In certain procedures for radiotherapy of the thorax, patient breathing has an effect on the procedure. Motion of the lungs and diaphragm can cause displacement of organs and a tumor being treated. Organs and tumors in the thorax and abdomen are known to move by more than 2 cm during the breathing cycle. At present, the 3D imagings used for treatment planning are “static”. They do not contain information about the changing tumor positions while the patient breathes. Consequently, a wide margin is used, irradiating a large volume of critical tissue. As a result, limits are placed upon the dose that can be delivered to the tumor. Concern for pulmonary complications has constrained radiation therapy of lung cancer, despite the dismal prognosis of the disease. High dose conformal therapy in the thorax and abdomen is more effective when organ motion due to breathing can be minimized.
There have been different approaches to minimizing respiratory motion. One approach is to have the patient shallowly breathe pure oxygen. Another approach has been through a technique known as “triggering” or “gating” in which the respiration cycle is monitored using an external device such as a spirometer or a string-gauge to turn on the beam only at a certain point in the respiration cycle. A possible component of this technique is to train the patient to exercise the breath-holding at the appropriate lung volume in order to extend the duty cycle of the beam. A further approach is to use deep inspiration breath holding, during which time the beam is activated.
The optimal delivery of gated or “breath-hold” therapy requires the 3D characterization of dynamic organ and tumor motion such that both beam geometry and “gate” can be optimized. However, this optional approach is not possible with most gated therapy proposals which rely on 2D fluoroscopy. It is also difficult to obtain gated 3D CT scans because of the complexity in machine control. Deep inspiration breath hold can be applied, but the 3D CT scan can only be made in one respiratory position. It is possible that dynamic 3D tomographic images can be made with the Immatron (an ultrafast CT specifically built for cardiac scanning) or using a fast MRI. However, the former approach is prohibitively expensive, while the latter approach produces distortions and complex image fusion is required to provide 3D images.
Accordingly, current approaches to gated therapy rely exclusively on the passive monitoring of respiration, followed by electronic or manual triggering of the beam. However, electronic triggering requires control of the medical accelerator to coordinate with passive respiratory monitoring. This is not readily achieved. On the other hand, manual gating requires the patient to reproducibly get to the same respiratory position. Inevitable variability means that a wider tolerance would need to be set. In addition, the radiation needs to be turned off immediately when the breath-hold creeps out of tolerance. Failure to do so can be serious since gated therapy is likely to employ higher dose rates.
While the above techniques represent various advances in the art, all known methods and devices for the delivery of radiation therapy during suspended ventilation are subject to improvement.